Nils Diaz, chairman of the Nuclear Regulatory Commission, said Monday he will leave the agency at the end of June when his five-year term expires.

Diaz, a nuclear engineer, has been on the commission for 10 years and was named its chairman three years ago by President Bush.

"I plan to return to Florida after my second term expires on June 30 and enjoy time with my family," Diaz said in a statement.

Diaz has been head of the five-member commission during a time when it has faced a wide range of challenges, from developing new security requirements amid heightened concerns about terrorism to preparing to license the first new nuclear power plants since the 1970s.

The president can promote one of the four remaining NRC commissioners to the chairmanship or nominate a new member as chairman, requiring Senate confirmation.

Before coming to the NRC, Diaz was a professor of nuclear engineering science at the University of Florida. He was also director of the Innovative Nuclear Space Power Institute, a consortium of industries, universities and national laboratories.

The NRC regulates commercial nuclear power plants and nuclear material.